A priori, this would seem to be a solder mask problem, but it is not 
possible to give you a cause. Some of the possibles are (non exhaustive):
1. Poor curing of the mask
2. Non-stoichiometry of the mask
3. Non-homogeneity of the mask
4. Mask past sell-by date
5. Mask applied over hygroscopic contaminant
6. Hydrogen bonding between flux component and mask
7. Inadequate cleaning

Now, many of these are inter-related (e.g., 6. requires one of 1. to 
4.). Whatever, except for 7., you need to intervene at a PCB Fab level.

Brian


Charlie Pitarys wrote:
> I know this has been brought up several times. Searched the archives but 
> more help will be appreciated.
> Full process:
> 
> Before Reflow PCB have Temperature storage at 65°C for a minimun of 24H 
> (some time few weeks)
> 
> After Refow before wave solder the PCB have Temperature storage at 70°C 
> for 8 hours (the night)
> 
> After Wave solder (Pacific 9000M flux) 100% of production is cleaned. Current 
> wash temp. was set at 70C. Yes I know this is extreme and advised to 
> lower<60C. This helped a little.
> If a sample section PCB is protected with Kapton and Kapton removed before 
> cleaning No Problem appears on that solder mask area.
> 
> You can even see an outline where the wave solder fingers caused shadowing 
> of the solder mask and on the top side along the edge where some flux has 
> wicked over.
> 
> I need to direct my customer to documents, etc. that have substance on this 
> issue. He is hesitant to receive my advise and reasons that I know for this 
> occurance. Bad solder mask.
> The problem of white mark is only on few case (less than 5% of PCB).
> 
> The PCB I saw was subjected, before any process a Temperature storage of 
> 65°C during 3 weeks (can this make the PCB more sensitive ?)
> My flux analysis shows the spectrum has all the signature features for a 
> carboxylic acid and a library search brought up adipic acid as the closest 
> match.
> Thanks in advance
> 
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